Polyacetal resins not only have an excellent balance of mechanical strength, chemical resistance and sliding properties, but can also be easily processed. By virtue of these advantages, polyacetal resins are widely used, as engineering plastic materials, for a wide variety of applications including mechanical parts, such as mechanical parts for electric and electronic equipment as well as automobiles.
When a polyacetal resin is used in the above-mentioned fields, it is necessary that the polyacetal resin have good thermal aging resistance and aging discoloration resistance. Further, from the viewpoint of improving molding productivity, it is important that the polyacetal resin have good anti-mold deposit properties.
Conventionally, as a method for improving the thermal aging resistance of a polyacetal resin, it is generally known to add an aliphatic carboxylic acid metal salt to the polyacetal resin see, for example, Examined Japanese Patent Application Publication Specification No. 55-22508 (corresponding to U.S. Pat. No. 3,743,614) and Examined Japanese Patent Application Publication Specification No. 60-56748 (corresponding to GB Patent Application Publication No. 1425771)!.
However, when a conventionally known aliphatic carboxylic acid metal salt is added to a polyacetal resin, although the thermal aging resistance of the polyacetal resin can be improved to some degree, the aging discoloration resistance and anti-mold deposit properties of the polyacetal resin become low. The poor aging discoloration resistance causes a molded product to have a poor appearance. The poor anti-mold deposit properties of the polyacetal resin lead to a lowering of the molding productivity, because a set of mold halves used for molding such a polyacetal resin needs to be frequently dismantled and cleaned. It has been strongly desired in the art to solve these problems.